The present invention relates to a device for washing and dewatering a fibrous suspension.
In the production of pulp from cellulose containing fibrous materials there are one or several points in the process where there is a need to wash and de-water the pulp.
A known and habitually used device for the washing and dewatering of pulp is disclosed in SE-C-380 300. The device shown therein utilizes two cylindrical, rotatable filter drums arranged in an essentially converging trough, which however is partly diverging at the inlet for the wash fluid. Characteristic of this existing type of device, as shown in SE-C-380 300, is that the trough normally extends round only about 180xc2x0 of the filter drum""s circumference, even if FIG. 1 in the description shows a gap for the pulp between the filter drum and the trough (not shown), which seem to extend somewhat further over the filter drum""s circumference. This implies that effective dewatering of the pulp can only be achieved under a relatively limited sector of the filter drum""s circumference, since effective dewatering only takes place where the pulp is pressed by the trough walls against the filter drum.
SE-C-501 710 seems to disclose a further development of the device in SE-C-380 300, and deals principally with the sealing aspect. The same applicant also has U.S. Pat. No. 5,488,900 and SE-C-504 011, in which the US patent discloses a device with pulp inflow arranged at the bottom, whereas the Swedish patent discloses a simplified arrangement, which is not designed to wash the pulp and which gives a relatively low dryness of the out flowing pulp, where it is also is said that a certain re-wetting of the dewatered pulp flow is inevitable.
Other examples of known devices are disclosed in U.S. Pat. No. 4,543,161 and U.S. Pat. No. 5,667,642 in which the latter shows a device where the drum rotates in the opposite direction to that conventionally used, i.e. seen from the end face the right drum rotates anticlockwise and the left clockwise.
For wash presses with only one filter drum it is known to arrange a trough which extends round a larger sector of the filter drum""s circumference and which thereby provides a longer effective dewatering zone. See for example U.S. Pat. No. 4,986,881 where however cleaning means to flush away remaining fibre residues on the filter drum is missing. U.S. Pat. No. 4,085,003 and U.S. Pat. No. 5,046,338 also show embodiments with only one filter drum.
In SE-C-318 182 (CA,A,862450) a wash press is shown with one hollow filter drum (FIGS. 1 and 2) and also a variant with two hollow filter drums (FIG. 3) where in the latter case one filter drum is arranged above the other filter drum. Here information about means for continuous cleaning/flushing away of remaining fibre residues on the filter drum is missing. In the practical implementation of the designs with one filter drum (FIGS. 1 and 2) flushing nozzles have been installed immediately after the removal of the dewatered fibrous web. The pulp inflow chamber (detail 3) has in that case been limited in the direction of rotation of the filter drum so that a space is found for these nozzles. Consequently, the water which is led down does not adversely affect the dewatering function as the water only splashes on the pulp inflow chamber containing pressed and not dewatered pulp. Normally, the process water is not sprayed on the pulp in the pulp inflow chamber where the pulp is put underpressure. The variant with two filter drums (FIG. 3) has not resulted in a commercial product, partly due to the fact that a good solution for continuous flushing of the remaining fibre residues has not been found for the upper filter drum. If the flushing liquid from the nozzles is to be prevented from wetting the dewatered pulp, an extensive deflection plate must be installed over the press roller and conveyer screw as well as over the whole width of the dewatering press, with the objective of collecting this flush water. In the embodiment shown the cutting plough share and inflow sealing are integrated in one and the same part.
In U.S. Pat. No. 5,421,176 a further alternative to the solution is disclosed, in which a cylindrical, hollow filter drum cooperates with a solid press roller (detail 32). Here the pulp web extends over about 300xc2x0 and the flushing away of the fibre residues is achieved with a spray (detail 52) arranged alongside the filter drum""s descending surface. These sprays are often found in a specific number distributed over the filter drum. Normally the flushing water is supplied to the sprays at a pressure level of about 2-8 bar.
A problem with devices according to the known technology, with two contra-rotating cylindrical filter drums, is that effective dewatering only takes place on a relatively limited section of the circumference of the filter drum, normally less than about 180xc2x0. Despite the fact that this type of device has been known for decades and that longer effective dewatering zones have been known for a considerable number of years for devices with only one filter drum, nobody has been able to realize a working concept which incorporates a long effective dewatering zone for equipment with two filter drums.
A second problem is how to clean the filter drums continuously during operation. For this purpose sprays have been used which use water jets to remove remaining fibre residues. In certain cases it is desired also that the holes in the filter drum are cleaned of deposits. This calls for high pressure nozzles to be used working at pressures over 200 bar and as high as 2000 bar. Thus, in U.S. Pat. No. 5,421,176 and in devices with two filter drums (for example U.S. Pat. No. 4,861,433) sprays are arranged on the filter drum""s descending side. This enables the flushed-off fibres to fall off and ensures that the dewatered pulp is not wetted by the water from the sprays. Another alternative for cleaning sprays is disclosed in U.S. Pat. No. 5,667,642 where the sprays are arranged below the filter drum. Here the flushed-off fibres can fall off and away from the filter drum.
A related problem is to achieve a trough construction which encloses a large section of the filter drum, circumference and which is still easy to displace in relation to the filter drums for cleaning and washing of the filter drums and the space between the trough and the filter drums.
One objective of the invention described here is to offer an improved dewatering capacity. This is achieved by means of double filter drums with a fibrous web round at least 230xc2x0, and by completing the fibrous web or webs with a pres pinch in which evacuation of fluid takes place in the pinch in both directions in the filter drums"" interior.
A further objective is to provide additional washing zones in a dewatering device.
Another objective is to construct a more effective device for the washing and/or dewatering of a fibrous suspension which provides increased capacity or alternatively a smaller device with the same capacity when compared with the larger plant based on known technology.
Another objective is to obtain a device for washing and/or dewatering where a very high degree of initial dewatering can be obtained.
Another objective is to be able to clean the filter drum part continuously during operation without wetting the dewatered pulp and so that the flushed-off fibres can be channeled away.
According to one aspect of the invention the trough is installed to enclose the outer surface of the filter drum from the inflow chamber and further round at least 230xc2x0, preferably at least 245xc2x0 and most preferably at least 260xc2x0 of the circumference of the outer surface, so that during operation the fibrous web is constrained to run between the outer surface of the filter drum and the trough at least 230xc2x0, preferably at least 245xc2x0 and most preferably at least 260xc2x0 of the circumference of the outer surface before the fibrous web reaches the press pinch.
According to another aspect of the invention the pulp inflow chamber is installed at xe2x88x9220xc2x0 and 40xc2x0, preferably at xe2x88x9210xc2x0 and 30xc2x0, more preferably at 0xc2x0 and 30xc2x0 or most preferably at 0xc2x0 and 20xc2x0 round the filter drum, where 0xc2x0 denotes the filter drum""s highest point and a positive increase in degrees is reckoned in the direction of rotation of the filter drum. The pulp inflow chamber itself can be installed somewhat after the highest point of the filter drum but have a distribution chamber where distribution of the pulp fed out occurs to a certain extent contrary to the direction of rotation of the filter drum and towards its highest point.
According to a further aspect of the invention the trough contains an upper trough segment which encloses the outer surface of the filter drum from the pulp inflow chamber to an end point which is located approximately 90xc2x0 to 130xc2x0, preferably 110xc2x0 to 120xc2x0 from the pulp inflow chamber, reckoned in the direction of rotation of the filter drum. Further, the upper trough segment is suitably pivotable about an axle 9, which is parallel to the outer surface of the filter drum and is installed close to the said end point 11, preferably at a maximum of 30xc2x0 from the said end point. Thus the pivoting can take place at a distance from the pulp web as shown in the figure and in certain cases displaced relative to the angular extent of the filter drum. The trough also includes a lower trough segment which encloses the outer surface of the filter drum from the upper trough segment""s end point trough to the pinch and which trough segment can be lowered.
According to another aspect of the invention the device can either include two stationary pulp inflow chambers, one for each filter drum, whereby the device is in the main symmetrical in a symmetry plan which is based on a tangent to the filter drum in the pinch, or can consist or a principal filter drum equipped with a pulp inflow chamber and converging trough, while the second filter drum provides a perforated press roller with internal evacuation chambers for increased dewatering in the pinch, and where the said second filter drum preferably has a smaller diameter than the first filter drum.